Breathable polymer films have the ability to be waterproof, yet be permeable to water vapor. This property makes the films useful in such applications as house wrap, and for breathable protective wear—such as for sporting use, medical use, military use, shoes, and protective bedding. The films may be applied to woven and non-woven materials for permanent or disposable use.
Breathable mono-layer films can be produced using polyether-block-polyamides (PEBA), and polyetheresteramides, as described in U.S. Pat. Nos. 5,800,928; 5,869,414; and US 2004/0029467. Unfortunately PEBA polymers have very poor adhesive properties. In order to improve adhesion, some amount of a polyolefin-based polymer is blended with the PEBA to form a film that can be bonded to a woven or non-woven material by the use of an adhesive, or by heat lamination. The films should have good drawability and flexability, and be easily extruded. The films in the art are a blend of the block copolymer with a copolymer of ethylene/alkyl(meth)acrylate that may optionally be functionalized. In addition to improving adhesion, the ethylene/alkyl(meth)acrylate improves processability and the appearance of the film. Diluting the more expensive PEBA with a cheaper polyolefin also reduces cost. The down-side to adding the polyolefin is that it reduces the breathability of the film, thus the blends of the PEBA and the polyolfin must be properly balanced for optimal final properties.
A multi-layer house wrap is described in EP 1,650,022 having a non-woven layer bonded to a moisture control layer, a tie layer of ethylene vinyl acetate, and a layer of copolyetheresters.
US 2010/0003486 describes a DEET resistant PEBA film, and articles made from it by lamination or adhesion. The hydrophilic polyether block is believed to provide breathability, while the polyamide block provides water-proof properties. Two or more different PEBA copolymers, having different ratios of polyether blocks to polyamide blocks can be blended to optimize the breathability and water-resistant properties.
Mono-layer films combining PEBA with ethylene/alkyl(meth)acrylate for housewrap applications generally combine about 30 wt % of an alloy of two different PEBA copolymers having different ratios of PE to PA blocks, with about 70% of ethylene/alkyl(meth)acrylate. The highly breathable PEBA (high polyether block) unfortunately lacks good processibility, and must be combined in an alloy with a less breathable, but more processible, PEBA having a higher polyamide block percentage.
The present invention solves the problem of producing a very highly breathable film, while also providing good adhesion onto a polypropylene or other substrate, by using a multi-layer film having at least one thin layer with a blend of a polyolefin and a breathable resin, and a second thicker layer having a high level of breathable resin with a lower level of polyolefin. When a polyether-block-polyamide (PEBA) copolymer breathable resin is used, the PEBA used can have a high level of polyether blocks for excellent breathability, while the film is easily processable in an extrusion or coextrusion coating process. The thinner layer(s) with a higher level of polyolefin serves as a bonding layer with a woven or non-woven substrate; while the thicker, mostly PEBA layer provides highly breathable, water-resistant layer.